1. Field of the Invention
The invention relates generally to systems and methods for use in the treatment of congestive heart failure, and, more particularly, to systems and methods for measuring a patient's heart rate response to respiration for monitoring the patient's autonomic function.
2. Description of the Related Art
Cardiac pacemakers generally provide functions including sensing electrical signals generated by the heart, controlling stimulation of excitable tissues in the heart, sensing the response of the heart to such stimulation, and responding to inadequate or inappropriate stimulus or response, e.g., dysrhythmia, to deliver therapeutic stimuli to the heart. Some existing cardiac pacemakers also function to communicate with an external programmer device to support a variety of monitoring, diagnostic and configuration functions.
Certain cardiac pacemakers include internal sensors which can provide information valuable for this purpose, such as a transthoracic impedance sensor. This sensor can be used to monitor the patient's respiration. If the person's minute ventilation becomes high enough, it is an indication that their heart rate needs to be increased to meet metabolic demand.
Cardiac Resynchronization Therapy (CRT) is a relatively new but promising therapy for the treatment of congestive heart failure (CHF). CRT is a method of applying cardiac pacing therapy in a failing heart, that establishes more normal ventricular wall and atrial-ventricular synchrony. In CRT, a pacemaker may be implanted to restore synchrony to the beating of the heart, thereby increasing the heart's pumping efficiency.
CHF patients represent a large and growing population of patients that can benefit from implantable devices. CHF as a disease comprises a complicated set of interrelated systemic dysfunctions, involving the cardiac and circulatory systems, the autonomic system, renal system, and the respiratory system.
Due to the complex nature of CHF, in order to understand and track the progression of the disease, it is necessary to monitor a variety of symptoms presented by the affected systems. An implanted device is in a unique position to aid the CHF physician because of its ability to monitor a number of the symptoms of CHF over extended periods of time. In particular, many implanted devices are able to monitor heart beat intervals on a beat-by-beat basis, and some implanted devices also are able to monitor respiratory patterns of the patients.
Respiratory Sinus Arrhythmia (RSA), a phenomenon that has been known to physicians for centuries, describes the change of heart rate with respiration. Generally, in healthy persons, resting heart rate increases simultaneously with inspiration, with the magnitude of the effect being directly related to the depth of respiration, and also somewhat affected by the respiration rate. The phase of the response is also somewhat affected by the respiration rate. In general, RSA decreases with age, and diseases such as CHF can lead to greatly diminished RSA. Studies have shown RSA to be mediated predominantly but not entirely by vagal control of the heart.
A large portion of a healthy person's heart rate variability (HRV) is due to RSA. In fact, the high frequency (HF) component is almost entirely due to RSA. Since RSA contributes significantly to overall HRV, and in fact dominates the HF portion of HRV, and since RSA is generally proportional to the depth of respiration, it follows that the depth of respiration will to some degree affect many measures of HRV, particularly those specifically dependent on the HF part of the HRV spectrum. Abnormal respiration patterns may affect all components of HRV.
RSA is potentially a more accurate measure of vagal tone than HRV which analyzes heart beat intervals without any attempt to quantify or distinguish between the many factors that may be responsible for heart rate (HR) fluctuations. Measures of autonomic function based on HRV alone may be limited in accuracy and specificity because they can be affected by variable and/or abnormal respiration patterns. In particular, shallow breathing due to congestion may itself lead to a reduced high-frequency HRV component unrelated to the autonomic tone of the patient. RSA is a ratio which is a specific measure of the response of HR to a given input (respiration). Further specificity may be provided by monitoring and correcting for activity, posture, and other variables.
Hence, those skilled in the art have recognized a need for providing a system which is capable of monitoring a patient's autonomic function, by RSA measurement of the patient's heart rate response to respiration. The invention fulfills these needs and others.